This invention relates to articles for use in high temperature environments. More particularly, this invention relates to articles for use in high temperature, oxidative environments, such as, for example, a gas turbine assembly. This invention also relates to methods for manufacturing such articles.
A number of industries, such as power generation and aerospace, demand equipment for prolonged service at high temperatures and oxidative environments. Equipment such as turbine assemblies, including, for example, aeronautical turbines, land-based turbines, marine-based turbines, and the like, typically include components formed from a class of materials known as superalloys. Superalloys exhibit desirable chemical and physical properties under the high temperature, high stress, and high-pressure conditions generally encountered during turbine operation. Nickel (Ni)-, iron (Fe)-, and cobalt (Co)-based superalloys are of particular interest in such applications.
Turbine assembly components, such as turbine blades, vanes, and combustion components in modern jet engines, for example, often reach temperatures as high as about 1,150° C., which is about 85% of the melting temperatures of most Ni-based superalloys. At such high service temperatures, the superalloys that are used to form the components are highly susceptible to damage from such mechanisms as creep, oxidation, and melting. Thermal barrier coatings are often applied to the surface of superalloy components to afford some means of insulating the metal from the hot gas. The resulting reduction in metal temperatures increases the effective component lifetime at a given operating temperature, or increases the operating temperature a component may be exposed to for a desired lifetime.
Thermal barrier coatings (TBC's) typically comprise ceramics such as yttria-stabilized zirconia. Prior to applying a thermal barrier coating to a superalloy component, a metallic bond coat is generally deposited on the superalloy substrate to provide enhanced protection against oxidation to the superalloy surface. Conventional bond coats used on components exposed to the hot gases of combustion in gas turbine engines include both diffusion aluminides and MCrAl(X) coatings. The term “aluminides” encompasses a wide variety of coatings comprising aluminide compounds of various chemical compositions. For example, nickel aluminide, NiAl, is often grown as an outer coating on a nickel-based superalloy by exposing the superalloy substrate to an aluminum-rich environment at elevated temperatures. The aluminum diffuses into the substrate and combines with the nickel to form a coating of NiAl on the outer surface. A platinum-containing nickel aluminide coating is often formed by electroplating platinum over the nickel-base substrate to a predetermined thickness, followed by exposing the platinum-coated substrate to an aluminum-rich environment at elevated temperatures. In addition to aluminide coatings, MCrAl(X) coatings, where M is at least one of Ni, Co, and Fe, and wherein X is at least one of yttrium (Y), tantalum (Ta), silicon (Si), hafnium (Hf), titanium (Ti), zirconium (Zr), boron (B), carbon (C), are commonly used as bond coats for a TBC system. MCrAl(X) coatings are suitable for application by any of a number of processes, including plasma spraying, high-velocity oxy-fuel (HVOF) spraying, and physical and chemical vapor deposition, as non-limiting examples.
Both the aluminide and MCrAlX bond coats comprise a significant amount of aluminum, and in the case of MCrAlX, a significant amount of chromium as well. During exposure to high temperatures in an oxidative environment, these elements in the bond coat provide oxidation resistance to the substrate by forming a thin, compact, and tightly adherent layer of oxide scale at the interface of the bond coat with the ceramic thermal barrier coating. This scale, also referred to as a “thermally grown oxide” or “TGO,” significantly reduces the ability of oxygen to diffuse into the coating to attack the substrate, thereby inhibiting substrate oxidation.
Although effective in extending the high-temperature capability of superalloy components, both the aluminide-with-TBC coating system and the MCrAlX-with-TBC coating system exhibit limitations which continue the need for improved materials for high temperature applications. For example, with time, the aluminum and chromium present in the bond coats become substantially converted into TGO, whereupon the protective capability of the bond coat is exhausted. At this stage the component is susceptible to damage, and thus components are periodically inspected and often repaired or replaced, based on the condition of the coating system and substrate material. This inspection and refurbishment is often time-consuming and expensive, and thus there is a need for coatings, especially bond coats, with enhanced ability to protect the substrate from oxidation. In addition, as the TGO layer grows at the interface of the bond coat with the TBC topcoat, spalling of the TBC often results, causing an immediate rise in temperature, and accelerated degradation, in the spalled area. Thus there is a need for articles and coating systems that resist spalling of the thermal barrier coating. Finally, in advanced turbine assembly design concepts, the surface temperatures of components are expected to exceed the melting points of state-of-the-art superalloys. Therefore, a further need is for articles with enhanced ability to withstand exposure to high temperatures.